DBV-200 Hard Failure Code 4

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Hello all. New user here.  Fantastic site!

I'm trying to repair my father's SG-112B, and I've narrowed it done to the DBV-200.  Using info from this site, I've been able to narrow the problem down a little further.  On enabling after power up of the head alone in test mode (test #8- bill acceptance), the motor runs in reverse about 8 times then quits, giving me a hard error 4- jam in the bill path.  It will not accept a bill.  Checked the sensors visually and with test #3- manual validator sensor test, and all sensors (at least the two entrance and the HPL/HPR sensors that I can test) indicate normal passage of a bill manually passed through the head.

The head is a DBV-200 A344 with a tag indicating DBV-200-SH with ID 011/015.

The sensors are clean and clear, and I reseated the head numerous times.

Any suggestions?